We just visited these parks in May. We stayed at Gros Venture campground in GT and loved the location to explore both GT and Jackson. We have stayed at Coulter Bay before, and while it is more centrally located for the park, it was not yet open for the season this time. We then moved to Grizzly RV Park in West Yellowstone. ( check their rates as early and late season they drop them 50%) The figure 8 road in YNP is 200+ miles. The advantage to staying in WY is that it borders the west side of the park. We spent a day touring the upper half and one touring the lower. To exit the park we traversed YNP going east and stayed a few days at Buffalo Bill SP west of Cody. Awesome park, but it is a good 1 1/2 to 2 hour uphill windy drive to the eastern edge of YNP. Whatever you decide, enjoy your time in the parks, they are AMAZING!